The object of the present invention is an apparatus used for processing silicon wafers, the apparatus comprising two or more reactors of essentially different type with respect to each other, the reactors being suitable for processing the various processing stages of silicon wafers, such as the epitaxial growth of silicon, plasma etching or metal deposition under vacuum, that is, for processing silicon wafers in two or more essentially different processing stages. The invention also relates to a method for processing silicon wafers in an apparatus comprising two or more reactors of essentially different type with respect to each other, for two or more essentially different types of processing of silicon wafers taking place in succession, such as the epitaxial growth of silicon, plasma etching or metal deposition.
Semiconductor technology has advanced considerably during the past three decades. The production of single transistors has been replaced by the processing of silicon wafers. One of the most significant recent developments in silicon technology has been the reduction in element dimensions which has made it possible to pack several million transistors in one circuit. At the same time, the size of silicon wafers has increased from a few centimeters up to 20 cm, which means that a considerably greater number of more efficient circuits can now be fitted on a single wafer.
In currently known furnaces or reactors intended for processing silicon wafers with a view to mass production, several dozen, or even a few hundred wafers can be processed simultaneously. To meet the continually growing commercial demand for silicon wafers, so-called cluster-type processing systems have been developed, in which several completely independent reactors used for typically similar processing of silicon wafers are connected around one common silicon wafer loading chamber. In this way it has been possible to increase the production of silicon wafers considerably.
The American patent no. U.S. Pat No. 4,951,601, for example, discloses a silicon wafer processing system which enables several different silicon wafers to be handled simultaneously at different processing stages. The system comprises several reactors to be used in the different phases of processing silicon wafers, the reactors being arranged in a cluster-like fashion around a common silicon wafer loading chamber. In this system, the silicon wafers are conveyed through the loading chamber, from one reactor to another, that is, from one processing stage to another. All the reactors operate simultaneously, and thus each is equipped with its own vacuum system and gas feed and control system as required by each process, to establish the required different conditions in each reactor.
The price of semiconductor processing apparatus has always been high. As the requirements for the accuracy and stability of temperatures have become more stringent, the prices have risen even further and are now beyond the means of ordinary universities and many research institutes.
To be able to give students at least some idea of the processes, a general solution has been to acquire used equipment suitable for handling small-diameter wafers and to lower the standard of the experiments. In this case, the apparatus of each processing stage thus typically forms a complete unit of its own, including a reaction chamber, its own separate automation, its own separate vacuum and heating equipment and gas distribution pipelines and valves. The number of pieces of equipment is thus considerable and the cost of the total equipment high. Such large equipment entities also require considerable clean room space in teaching or research institutes. The aim of the present invention is, therefore, to provide an apparatus intended for processing semiconductors, especially silicon wafers, where the above drawbacks have been minimized.
The aim of the invention is more particularly to achieve a compact apparatus, versatile in use and reasonable in price, for processing silicon wafers, especially for teaching and research purposes.
In order to achieve the above aims, the apparatus and method relating to the invention for processing semiconductors, particularly silicon wafers, are characterized by what is specified in the claims below.
In the apparatus used for processing silicon wafers according to a preferred embodiment of the invention, two or more reactors of essentially different types are connected to a common central unit which controls the functions of the apparatus. The reactors operate in succession, so that while one reactor is in operation, the others are at a waiting stage. Each reactor is typically used when it is operating in different conditions, for example, in a different pressure range, at a different temperature or in a different gaseous atmosphere than another reactor operating at a different time.
In the apparatus relating to the invention, the central unit comprises, for example, vacuum equipment common to all reactors, such as a vacuum pump and/or a turbo pump, to which the reactors are connected by means of vacuum valves that can be closed separately. During the waiting stage, a high vacuum is created in the reactors and the reactors are cut off from the system by valves. In the operating reactor the pressure is increased to the suitable level.
The central unit also includes a gas distribution and control system common to all the reactors. The reactors are connected to the gas distribution system by gas valves which can be closed and controlled separately, by means of which valves the reactors at the waiting stage at a given time can be cut off from the system, and by means of which the feeding of gas to the reactor activated at any given time can be controlled. For example the following gases are used in processing silicon wafers: gas containing oxygen, nitrogen, hydrogen, silane, arsine and/or phosphine. Gas pipelines are provided separately for each gas, the pipelines being connected to that reactor or those reactors which use the gas in question. The flow of gases is controlled by a common gas control system.
The central unit preferably also includes an electricity distribution system common to the reactors, and a common high-frequency alternator. In addition, the reactors preferably have a common silicon wafer loading chamber, through which the silicon wafer is transferred to the reactor desired at a given time. The loading chamber is preferably connected to the vacuum equipment of the central unit to create a negative pressure or vacuum also in the loading chamber. The negative pressure reduces to a substantial degree the mixing of different gas spaces in the system.
In applying the method relating to the invention, one or possibly several, silicon wafers are processed in only one active reactor at a time, the reactor active at a given time being connected to a common vacuum pump system and the other reactors being cut off from it, that is, at the waiting stage. The gas required for each processing of the silicon wafer--such as gas containing oxygen, nitrogen, hydrogen, silane, arsine and/or phosphine--is conducted, by controlling the common control equipment, only to the reactor active at a given time. During the waiting stage the other reactors are kept under a high vacuum, preferably under a vacuum of about 10-100 Pa.
When the development of the processing of silicon wafers in recent years is reviewed, the dimensions of integrated circuit elements nearing the .mu.m limit, it may be noted that long heat treatments lasting for hours at temperatures ranging between 1000-1200.degree. C. are no longer required. The temperatures used are lower and processing times shorter and, therefore, from the point of view of use of time it is often not important to be able to process several silicon wafers simultaneously, but they can be produced in smaller reactors intended for only one silicon wafer, one wafer at a time.
Other substantial changes relating to the processing of silicon wafers have also taken place. Etching in acids has for the most part been abandoned. Etching now often takes place in gaseous plasma in plasma reactors. Films are grown by synthesizing from gases, often at lowered pressures, in so-called CVD (Chemical Vapour Deposition) or LPCVD (Low Pressure Chemical Vapour Deposition) reactors. In addition, increasingly often, instead of standard circuits for general use, Application Specific Integrated Circuits (ASIC) or the client's own full- or semi-custom integrated circuits are produced, where series are small and the processing of one wafer at a time, as described in the present invention, may suffice.
Especially in teaching and research use in universities and many research institutes where there is no need for serial production, it suffices to handle one wafer at a time and to carry out the different stages of the processing sequentially, that is, in succession in different reactors.
In order to be able to study, for example, the production of integrated circuits or various detectors and sensors in as versatile a manner as possible, reactors of very different types are required. The number of reactors required can be minimized by designing each reactor to operate within as wide a range as possible. In modern semiconductor technology, reactors are required, for example, for the following types of processes:
Epitaxial growth of silicon and growth of polysilicon. PA1 Predeposition. The maximum temperature of the reactor is about 1100.degree. C.; the gases used are nitrogen, oxygen and, as doping gas, for example, phosphine, diborane or arsine. PA1 Oxidation and diffusion. The maximum temperature of the reactor is about 1100.degree. C.; the gases used are nitrogen and oxygen and water vapour. The reactor may have to withstand a pressure of up to 20 atm. PA1 LPCVD growth. The maximum temperature of the reactor is about 600.degree. C. and the negative pressure about 50 mbar; the gases used are nitrogen, oxygen and e.g. silane, ammonia vapour, aluminium and tungsten alloys. PA1 Plasma etching. Process pressure &lt;0.2 mbar and RF frequency e.g. 13.56 MHz; the gases used are e.g. CF.sub.4 or CCl.sub.4, and oxygen and hydrogen. PA1 Metal deposition under vacuum e.g. by sputtering. The maximum temperature of the reactor is about 300.degree. C. and the process pressure about 0.001-0.5 mbar. The reactor incorporates an aluminium or other metal cathode.
The epitaxial growth of silicon under low pressure requires a temperature of 500-800.degree. C., which is produced, for example, by resistive heating. A graphite sheet may be used as the heating element. The maximum temperature of the reactor is about 1100.degree. C., the growth pressure 10.sup.-4 -10 mbar (0.01-1000 Pa); the gases used are hydrogen, nitrogen and silane SiH.sub.4 or disilane SiH.sub.2 Cl.sub.2 and a doping gas.
Very different types of reactors are thus required to be able to carry out versatile research: reactors that withstand a vacuum or pressure, high temperatures or a corrosive gaseous atmosphere. For example, in the first three processes mentioned above, high temperatures make great demands on reactors. In the apparatus relating to the invention, a considerable number of different types of reactors can be combined, each of which is tailor-made only for a specific processing stage. On the other hand, the apparatus can obviously be made to consist of reactors operating within a wider range, in which case correspondingly fewer separate reactors are required. For example, the same reactor could possibly be used for epitaxial growth, predeposition, oxidation and LPCVD growth, but the pressure, temperature and gaseous atmosphere are adjusted each time to meet the requirements of the processing stage in question.
The structure and material of the reactors must be such that the required processing stages can be carried out in each of them reliably, cleanly and without subjecting the wafer being processed to thermal stress. In designing reactors for research use it may be presumed that the wafers have a diameter of 150 mm and lie flat in the reaction chambers with the active side up, in which case the diameter and height of the reaction chambers will range between 250-300 mm, depending on the maximum temperature. Stainless steel is a suitable material for reaction chambers in which a vacuum is required. Quartz and possibly an insulating outer covering which reflects heat must be used in the reaction chambers operating at the highest temperatures. The chamber may also be water-cooled.
In the apparatus relating to the invention, completely different types of reactors intended for processing silicon wafers can now be combined in an advantageous manner. Since the reactors operate in succession, they may be connected in turn to the reactors' common gas extraction, that is, vacuum system and common gas feed and regulation system and each reactor does not require separate systems as was the case before. In the apparatus relating to the invention, the high vacuum created for the duration of the waiting period ensures purity and prevents the gases from different processing stages from entering in the wrong reactor.
Considerable savings result from the use of the common central unit in comparison to previously known apparatuses, in which each different type of reactor used its own vacuum system and gas feed and control system, which constitute a major part of the total price of the apparatus.
The apparatus relating to the invention is also extremely compact and it requires little space compared to systems in which each reactor has its own vacuum, gas distribution, electricity distribution, automation and gas extraction systems. When ready for operation, the apparatus relating to the invention fits into a 20-30 m.sup.2 size room. An apparatus comprising three different reactors only requires a space of about 1 m.times.3 m.times.1.5 m, which means considerable saving of space compared to known systems.
The apparatus relating to the invention is intended primarily for the type of use in research and teaching, where the aim is to process silicon wafers in prototype fashion and to examine the different stages of processing, and where it is usually necessary to handle only one silicon wafer at one processing stage at a time. The apparatus relating to the invention can be set successively from the central unit to a state where only the reactor involved in the processing stage being examined at a given time is in operation and in contact with the devices controlling operation. The other reactors are at the waiting stage but can in turn be connected by means of valves and switches to the devices controlling operation.
The entire system is typically kept under a continuous vacuum. The reactors are connected to the connecting pipe of the common vacuum system and a high vacuum is created in the reactors by opening the vacuum valves between the reactors and the connecting pipe. After this, the reactors under a high vacuum and remaining at the waiting stage during processing are cut off from the system by means of vacuum valves, these reactors thus remaining under high vacuum for the duration of the entire waiting stage. Maintaining the high vacuum in the reactors during the waiting period ensures purity in the system.
After this, pressure in the reactor to be activated is increased to the level required by the processing. Pressure is regulated by adjusting the suction power of the turbo pump of the vacuum system, for example, by means of a throttle valve. The negative pressures or pressures required at the different processing stages vary to a great extent from one processing stage to another. The activated reactor is in direct contact with the turbo pump through the connecting pipe, which means that the pressure in the connecting pipe is always lower or--at its highest--the same as that in the activated reactor and thus there will be no gas flow from the connecting pipe to the reactor.
The apparatus relating to the invention, comprising two or more reactors and a common central unit, obviously also requires external patterning, that is, so-called masking equipment and possibly optical and electrical measuring instruments. Wet scrubbing is preferably also carried out outside the apparatus.